Chapter Description

From the Book

6-1: VLAN Configuration

VLANs are broadcast domains defined within switches to allow
control of broadcast, multicast, unicast, and unknown unicast within a Layer 2
device.

VLANs are defined on a switch in an internal database known as the
VLAN Trunking Protocol (VTP) database. After a VLAN has been
created, ports are assigned to the VLAN.

VLANs are assigned numbers for identification within and between
switches. Cisco switches have two ranges of VLANs, the normal range and
extended range.

VLANs have a variety of configurable parameters, including name, type,
and state.

Several VLANs are reserved, and some can be used for internal purposes
within the switch.

Creation of an Ethernet VLAN

VLANs are created on Layer 2 switches to control broadcasts and enforce the
use of a Layer 3 device for communications. Each VLAN is created in the local
switch's database for use. If a VLAN is not known to a switch, that switch
cannot transfer traffic across any of its ports for that VLAN. VLANs are created
by number, and there are two ranges of usable VLAN numbers (normal range
11000 and extended range 10254096). When a VLAN is created, you can
also give it certain attributes such as a VLAN name, VLAN type, and its
operational state. To create a VLAN, use the following steps.

Configure VTP.

VTP is a protocol used by Cisco switches to maintain a consistent database
between switches for trunking purposes. VTP is not required to create VLANs;
however, Cisco has set it up to act as a conduit for VLAN configuration between
switches as a default to make administration of VLANs easier. Because of this,
you must first either configure VTP with a domain name or disable VTP on the
switch. VTP is explained in detail in section "6-4: VLAN Trunking
Protocol."

NOTE

For Catalyst 4000 and 6000 switches running IOS Supervisor 12.1(8a) or above
(native IOS), you can configure the VTP parameters in global configuration mode
as well.

Specify a VTP name:

COS

set vtp domain domain-name

IOS

(vlan) vtp domain domain-name

-OR-

(global) vtp domain domain-name

By default, the VTP is in server mode and must be configured
with a domain name before any VLANs can be created. These commands specify the
VTP domain name. For IOS switches, you enter vlan database mode,
(vlan), by entering the command vlan database, at the
privileged-level prompt.

NOTE

The global configuration command vtp domain is not available on all
switches that run IOS.

-OR-

Disable VTP synchronization:

COS

set vtp mode transparent

IOS

(vlan) vtp transparent

-OR-

(global) vtp mode transparent

Another option is to disable VTP synchronization of the
databases. Disabling it enables you to manage your local VTP database without
configuring and relying on VTP. For Catalyst 4000 and 6000 switches running IOS
Supervisor 12.1(8a) or above (native IOS), you can configure the VTP parameters
in global configuration mode as well.

NOTE

The global configuration command vtp mode transparent is not available
on all switches that run IOS.

-OR-

Disable VTP:

COS

set vtp mode off

IOS

N/A

With the introduction of COS version 7.1.1, an option now
exists to disable VTP completely. Use the command set vtp mode off to
turn off VTP. After doing so, you can administer the local VTP database.

Create the VLAN.

VLANs are created by number. The two ranges of VLANs are as follows:

The standard range consists of VLANs 1 to 1000.

The extended range consists of VLANs 1025 to 4096.

Extended VLANs are currently supported only on switches running COS software
version 6.1 or greater. When you create a VLAN, you have many options to
consider. Many options are valid only for FDDI and Token Ring VLANs. Some of the
items configured deal with options, such as private VLANs, which are discussed
in other sections in this book. VLANs are created using the set vlan
command for COS devices or with the vlan command in vlan database
mode for IOS switches. For Ethernet VLANs, you can also configure the standard
parameters in Table 6-1.

Table 6-1 Configurable VLAN Parameters

Parameter

Description

name

A description of the VLAN up to 32 characters. If none is given, it defaults
to VLAN00XXX, where xxx is the VLAN number.

mtu

The maximum transmission unit (packet size, in bytes) that the VLAN can use;
valid values are from 576 to 18190. The MTU can extend up to 1500
for Ethernet, but beyond for Token Ring or FDDI. The default is 1500.

state

Used to specify whether the state of the VLAN is active or suspended. All
ports in a suspended VLAN will be suspended and not allowed to forward traffic.
The default state is active.

NOTE

Many other options are available during the VLAN configuration command;
however, most of these deal with the configuration of FDDI and Token Ring VLANs.
Because these are not widely used topologies, the options and descriptions of
Token Ring and FDDI VLAN configuration and parameters have not been included in
this book. For information on Token Ring or FDDI VLANs, refer to
http://www.cisco.com/univercd/cc/td/doc/product/ lan/cat5000/rel_6_3/config/vlans.htm.

The vlan-id specifies the VLAN by number. For COS you
can specify a range of VLANs in the vlan-id section; you cannot configure
the name for a range of VLANs, however, because each VLAN is to have a unique
name. For IOS switches, VLANs are created in vlan database mode. For Catalyst
6000 and 4000 switches running Supervisor IOS 12.1(8a) and above, you can create
VLANs in global configuration mode if the switch is in VTP transparent mode. To
do this, enter the vlan vlan-idcommand to move to
vlan-config mode. From vlan-config mode, you can manage the parameters of the
VLANs.

NOTE

You cannot modify any of the parameters for VLAN 1.

Create a VLAN in the extended range.

Extended VLANs support VLANs up to 4096 in accordance with the 802.1Q
standard. Currently only switches running COS 6.1 or greater can support
creation and assignment of VLANs in the extended range. You cannot currently use
VTP to manage VLANs in the extended range, and these VLANs cannot be passed over
an Inter-Switch Link (ISL) trunk link.

Enable spanning-tree MAC reduction:

COS

set spantree macreduction enable

IOS

N/A

To allow these switches to use the extended range, you must
first enable spanningtree macreduction to allow the switch to support a
large number of spanning-tree instances with a very limited number of MAC
addresses and still maintain the IEEE 802.1D bridge ID requirement for each
STP instance.

NOTE

After you have created a VLAN in the extended range, you cannot disable this
feature unless you first delete the VLAN.

Create a VLAN in the extended range:

COS

set vlanvlan-id [namename] [statestate] [mtumtu]

IOS

N/A

Here the vlan-id would be a number from 1025 to 4096.
Numbers 1001 to 1024 are reserved by Cisco and cannot be configured.

CAUTION

For Catalyst 6000 series switches with FlexWAN cards, the system identifies
these ports internally with VLAN numbers starting with 1025. If you have any
FlexWAN modules, be sure to reserve enough VLAN numbers (starting with VLAN
1025) for all the FlexWAN ports you want to install. You cannot use these
extended VLANs if you install FlexWAN ports.

Feature Example

In this example, the switches Access_1 and Distribution_1 are going to be
configured with VLANs 5, 8, and 10 with the names Cameron, Logan, and Katie,
respectively. Also the distribution switch will be configured with VLAN 2112
with the name Rush.

An example of the Catalyst OS configuration for Distribution 1 follows: